it stands for fuck it smile, a lifestyle created by Jay Anthony of the Diligentz, a Bay Area hip-hop group

"Well the abbreviation F.I.S can stand for multiple sayings such as; Fuck It Smile, Forget It Smile, Forever I'll Smile etc. The whole point of this brand new movement is to show people its okay to smile and its okay to be yourself.Life is way too short to trip off all this bullshit thats been going on, and i feel if you dont live by the rules of F.I.S life is going to be damn near too hard to accept.

MORAL: F.I.S..FUCK ALL THE BULLSHIT..DONT GIVE INTO IT..JUST SMILE AND MOVE ON...im not saying be a bitch, but some things just arent worth the trouble."-Jay-Ant

i'm livin that F.I.S. lifestyle, fuck all that bullshit, i'm just gonna make sure i'm happy.

Means to Fart In Self. When you're in a situation, like with a new boytoy (or fish) and you have to fart so bad that it goes up into your stomach making a loud rumbling, grumbling, farting sound. To which you say "hmmm I must be hungry" in a sad attempt to cover up.

Claire - Oh man, I was at Rusty's this morning, and my gas was so bad that I was Fissing like crazy!

the name you call a person who makes bad decisions and sleeps with a lot of people

a FIS is usually female
a FIS usually is a person who is or at one point was your friend

a FIS is a person including but not limited to:
-a 'friend' who has sex in front of you
-a 'friend' who has sex with the person who are fond of
-a 'friend' who makes fun of you for not having sex all the time
-a person who smokes TOO MUCH POT, smoking pot does not make them a fis but if it becomes ALL they talk about and do, then they just may be an FIS

1: "OH MY GOD, we were hanging out with that boy i like and she just fucked him, right in front of me!!!"